The present invention relates to a boost converter, and more particularly to a circuit to efficiently convert low voltages to a current for powering a light-emitting diode (LED).
LEDs are beginning to be implemented in environments previously reserved for that of incandescent lamps. One type of LED being used as such a replacement is a white LED, which is a blue LED having a phosphor that converts the ultra-violet or blue color to white. These white LEDs PROVIDE certain advantages over incandescent lamps, including having a lumens per watt rating of approximately 20, whereas an incandescent lamp of the same size will have a lumens per watt rating of 7-10. Further, similarly sized batteries will last for approximately twice as long when used with a white LED as opposed to an incandescent lamp. It is also known the white LEDs themselves will last longer at higher voltages than an incandescent lamp. For example, a incandescent lamp may last 50-100 hours, where a white LED may last as long as 10,000 hours or more at the same high voltage. White LEDs are also known to exhibit a brighter light output than other LEDs.
A drawback of white LEDs is that they have a larger band gap than other LED types. This larger band gap requires an operational voltage which is significantly higher than other LEDs, approximately 4 volts.
Approaches which have been used to supply the necessary voltage levels to a white LED include a d.c.-d.c. converter to regulate the output voltage. However, this approach is costly and is inefficient at low input voltages. Another approach has been to use a lithium cell to match the LED voltage directly. Again this is a costly and inefficient approach. Further, the settings in which the white LED light source of this application is intended to be used are low-cost implementations. For example, it is desired that the lighting system be able to be used with low power input such as two AA-size batteries. However, this battery combination is known to generate only approximately 1.5-3.1 volts. This voltage is of course not sufficient to operate the white LED.
Therefore, it has been determined that a need exists for an efficient circuit capable of transforming low battery voltages to an LED current sufficient to operate the white LED. Such a circuit must also be provided at a low component count and for small economic cost.
A self-oscillating boost converter includes a resistor-starting network configured to start a charging of the boost converter. A resonant feedback circuit is designed to generate an oscillating signal, following the starting of the circuit by the resistor-starting network. A complementary switching network has a pair of complementary common-source connected switches configured to receive the oscillation signal generated by the resonant feedback circuit. The oscillation signal determines a switching rate, or duty cycle, of the complementary pair of switches. A boost inductor is in operational connection with the complementary pair of switches. The switching rate of the complementary switching network acts to determine the boost voltage supplied to a load.